Electric wave limiting device



Nov. 3, 1931. E. PETERSON 1,830,240

' ELECTRIC WAVE LIMITING DEVICE Filed May 18, 1929 m4 5k 2 2 war? E6 8jfi/a g 598 /9 g f :li' (23 :llll I2? aurPz/T Y MA/v 30 [I] 3 38 QllvPl/T 5 33 1 33 39 II M534 wmv 701? E. PETERSON A r ram av PatentedNov. 3, 1931 PATENT OFFICE EUGENE PETERSON. OF NEW YORK, N. Y.,

ASSIGNOB TOIBELL TELEPHONE LABORA- TORIES, INCORPORATED, OF NEW YORK, N.Y., A' CORPORATION OI NEW YORK ELECTRIC WAVE LIMITING DEVICE Applicationfiled May 18, 1929. Serial No. 364,226.

This invention relates to wave transmission systems and moreparticularly to means for limiting disturbances in amplifier circuits.

In order to obtain high efficiency and maximum power from high poweramplifiers it is desirable to keep the input voltage as large aspossible without overloading the tubes or producing excess distortion.Provision must be made, however, for transients or other unavoidabledisturbances which when they occur might produce undesirable results.

An object of the invention is to limit the peak voltages impressed upona vacuum tube amplifier which are in excess of normal signal voltages.

Another object of the invention is to produce a sharp or abruptlimitation of both positive and negative voltage peaks.

In accordance with the present invention two amplifier tubes areconnected in cascade. Each tube has a resistance in the input circuitwhich prevents any increase in the plate current when the potential uponthe grid of the tube exceeds a predetermined value. When a Wave isimpressed upon the voltage limit ing device, it is faithfully re eateduntil a certain value is reached beyon which any increase in potentialof either polarity is unaccompanied by further increase of outputcurrent. The two stages in cascade are connected so that limitation ofone polarity takes place in the first stage while the other polarity islimited in the succeeding stage. The circuit thus permits both halves ofthe wave to have grid circuit limitations which results in much sharpercut-off than if plate circuit A better understanding of the inventionmay be had from the following description and the accompanying drawings,of which .Figs. 1 to 3 are difierent circuits embodying the inventionand Figs. 4 to 6 are curves illustrating their operation.

Referring to Fig. l, a pair of vacuum tubes 1 and 2 having similar inputand output circuits are seen connected in cascade through an interstagetransformer 3. The input c1rcuit of the tube 1 may be traced from thegrid 4 through resistance 5, the secondary Winding 6 ot' the inputtransformer 7, and

the source of grid potential 8 to the filament 9 of the tube. The outputcircuit of tube 1 may be traced from the late 10 through primary winding11 of tie interstage transformer 3 and a source of plate potential 13 tothe filament 9 of the tube. Similar circuits may be traced for thevacuum tube 2. Thus, the input circuit of the tube 2 may be traced fromthe grid 14 through the resistance 15, the secondary winding 16 of thetransformer 3, and the source of grid potential 17 to the filament 18 ofthe tube. The output circuit may be traced from the plate 19 through theprimary winding 20 of the output transformer 21, and the source of platepotential 22, back to the filament circuit 18. The filaments 9 and 18are energized, respectively, by the sources of energy 23 and 24.

The operation of the circuit may be readily understood by reference toFig. 4. The curve a is the grid-potential plate-current characteristicof one stage of the amplifier. When a normal voltage wave, such as thatrepre; sented by b is applied to the grid of tube 1, it is amplified andappears as a current wave 0 in the plate circuit of the tube. When avoltage wave, represented by d, exceeds a certain value determined bythe biasing potential on the grid, the latter element becomes positiveand grid current flows. However, when the grid is positive, the internalresistance between filament and grid is low, and resistance 5 is solarge compared with it that practically all of the potential drop occursacross this resistance, the grid potential going only slightly above itszero value. For th.s reason the plate current does not follow thevariations of the input wave when the positive cycle of the latterexceeds the negative grid potential 8 but is chopped off as shown bycurve e. The same phenomenonbccurs in the input circuit of. the tube 2,but the windings of the interstage transformer 3 are poled so that inthis circuit the other half cycle of the wave is limited.

Fig. 5 illustrates the phenomenawhich take place in different parts ofthe circuit when the input wave is in excess of the normal value. Thus Grepresents the input voltage as a sine wave havin peak values greatl inexcess of the grid biasing voltage, whic is represented by E This waveis cut off in the manner heretofore described so that the voltageimpressed on the grid of tube 1 is shown by H. 'The voltage appearing atthe high side of the transformer 3 has the DE component removed and isshown by I. This wave has the same shape as that shown by II, but ismoved up so that the area of the positive half cycle of the wave is .eual to the area of the negative half cycle. 11 account of theamplification occurring in tube 1, this wave, I, would have a greateramplitude than the grid wave, H, but for'simplicity in explaining theaction, the effect of the amplification has been neglected in thesecurves. The voltage Wave impressed upon the grid of the tube 2 with thenegative portion of the wave limited is shown b J. The portion of thewave that is limite in this stage is the negative portion of theoriginal input wave although as regards the tube 2 the part of the wavelimited in its grid circuit is positive on account of the shift in phaseoccasioned by the transformer 3. The final output wave is represented byK and has the DC component introduced in the plate circuit of the tube 2removed.

In order to repeat waves of the shapes indicated in Fig. 5, theinterstage and output transformers of Fig. 1 should have goodtransmission characteristics over the frequency band including all ofthe important components represented by the fiat topped waves, andshould also have a zero or a small phase shift over this range.

In the case of a radio transmitter, the power limiter of the inventionwould-be used to prevent damage to the high power transmitting tubes bypreventing excessive voltages from reachin them through the inputcircuit. The circuit ofFigs. 1, 2 or 3 would be placed in the inputcircuit leading to the high power tubes. In this case the antenna wouldpreferably have a relatively-narrow band frequency characteristic,incapable of transmitting the wide range of components which thetransformers of Fig. 1 (or t e Figs. 2 and 3 circuits) are designed totransmit, and this will reduce to a harmless amount the distortionarising from the flattening of the peaks of the waves as shown in Figs.5 and 6. v

Fig. 2 shows another embodiment of the invention in which the two tubestages are resistance coupled, thereby permitting both halves of thewaves to be limited without removing the DC components. This circuitpermits both halves of the waves to be cut off in exactly the samemanner. In Fig. 2 the input of the tube 30 is the same as the inputcircuit of tube 1 of Fig. 1, while the output circuit of tube 31,issimilar to the output circuit of tube 2. The output circuit of tube 30may be traced from the plate 32 through the resistance 33, the source ofplate potential 34- to the filament 35. The input circuit'of tube 31 maybe traced from the grid 36, through the resistance 37, source of gridpotential 38, the resistance 33, and source of potential 34 to thefilament 39.

In the operation of this circuit the potential which appears in theoutput of the tube 30 is opposite in phase to the voltage impressed uponits grid. Thevoltage appearing across resistance 33, containing a DOcomponent from the plate circuit of tube 30, is limited in the inputcircuit of the tube 31. By means of this circuit both halves of the wavemay be limited by precisely the same amount thus giving a somewhatbetter result than that obtained by the. circuit of Fig. l.

The phenomena which take place in different parts of this circuit areillustrated by Fig. 6. For instance, L represents the input voltage as asine wave. After limitation occurs in the input circuit of the tube 30in the manner heretofore described, the voltage impressed upon its gridis shown by M. The voltage in the output circuit of this tube due to thevoltage upon its grid is shown by Nand is exactly the same shapealthough opposite in phase to M. When limitation is effected in the gridcircuit of tube 31 the resultant voltage is shown by O and since the DCcomponent from theplate circuit of the tube 30 waspresent at the timethat limitation was effected both half cycles of the resultant wave areequal and symmetrical. The wave appearing in the output of this circuitis shown by P. In the case just described the resistance 33 of Fig. 2may be apportioned so that the input levels of tubes 30 and 31 areequal.

Fig. 3 shows another type of voltage limiter also employing resistancecoupling. In this circuit an attenuating circuit 50 is connected betweenthe tube stages, in order that the AC voltage applied to the grid of thesecond tube may exactly equal that applied to the first tube. Thispermits the limitation of both halves of the'wa-ve to be effected atzero gain, so that both halves of the waves are tubes 53 and 55 iseffected in the same way as in the circuits heretofore described. Thecurves of Fig. 6 are equally applicable to the said limiting operationof the circuit of Fig. 3 and of the voltages impressed upon the inputcircuit' of said devices from exceeding a predetermined value.

2. A voltage limiting device com rising space discharge devices in aplura ity of stages, a resistance in the input circuit of each of saiddevices for limiting only the positive voltage peaks impressed upon theinput circuits thereof to a predetermined value, and. means forassociating said tubes with each other'in such a manner that both peaksof the voltage wave impressed upon device are subjected to the limitingaction.

. 3. An electric wave limiting device comprisin grid control spacedischarge devices m a p urality of stages, a resistance in the inputcircuit of each of said devices for limiting only the positive wavepeaks impressed upon said input *circuits and means for connecting saidspace discharge devices in such a way that the wave is impressed uponthe grids of said devices in phase opposition.

' 4. An electric wave limiting device com-" prising grid control spacedischarge devices in a p urality of stages, a source of bia singotential in the input circuit of each of sai evices, means comprising aresistance for preventing the grids of said tubes from becoming positivewhen the wave applied to the input 0 said limiting device exceeds aredetermined value, and interstage coup ing means. for said spacedischarge devices for permitting that portion of a wave to be limited inone stage which was not limited in a preceding stage.

5. In a voltage limiting device. a pair of vacuum tubes connected intandem relation, means in the input circuit of one of said tubes fordproviding a negative .potential upon the gri with respect to thecathode thereof, means for impressing voltage waves upon said inputcircuit, means therein for. reventing said grid from becoming positivewith respect to said cathode, correspondin "means in the input circuitof the other of sai tubes, and means for impressin the volta wave aslimited by said first tu upon t e in at circuit of said second tube inoppositepo arity to said tube.

ing a value sufiicient to the voltage 6. In a voltage limiting device, apair of vacuum tubes connected in tandem relation, means in the inputcircuit of one of said tubes for providing a negative potential upon thegrid with respect to the cathode thereof, means for impressing a voltagewave upon said input circuit, a resistance therein havrovide a negativegrid bias when current ows in said input circuit equivalent to thevoltage by which the positive voltage wave exceeds the absolute value ofsaid first mentioned grid bias, and corresponding means in the inputcircuit of the other of said tubes, and means for impressing the voltagewave as limited by said first tube upon the input circuit of said secondtube in opposite phase relation to said 7. In a communication channel, aeak voltage limiting device comprising a t reeelectrode vacuum tubehaving in the input circuit thereof a resistance man times as great asthe input impedance 0 said tube at zero grid bias and a negative gridbiasing voltage approximately equal to the maximum voltage of thesignals normally transmitted, and a second voltage limiting device intandem therewith, poled to limit eaks not subject to the limiting action0 said first device.

8. In a' signal channel, a voltage limiting circuit comprising athree-electrode vacuum tube having an input circuit com rising aresistance large in comparison wit the input impedance of said tube whengrid current starts to flow, and a grid biasing ,volta proportioned tothe maximum voltage to carried normally by said channel to limit thepositive peaks transmitted to substantially said maximum normal voltage,and a second voltage limiting circuit in tandem therewith poled tolimitthe peaks not subject to the limiting action of said first circuit.

- first tube.

In witness whereof, I hereunto subscribe

